Cable television (CATV) and other communications systems use many electronic components. An example of one such component is the equalizer. Equalizers are used by the CATV industry to correct for signal loss that occurs as signals flow through the long lengths of coaxial cable, strung between telephone poles or buried underground, to various points where consumers are then able to access the signals. Coaxial cable offers some finite loss to the signals transmitted through it. In other words, if signal is injected at 100% power at one end at all frequencies, less than 100% of that signal will be recovered from the other end of the cable and varies depending on the frequency. The loss associated with the coaxial cable is not the same at all frequencies, and the loss typically increases as frequency increases. The result is that, after passing through a length of cable, the spectrum of radio frequency (“RF”) signals is stronger at low frequencies and weaker at high frequencies due to cable loss, which is known as signal slope. It is harder to process a full signal spectrum with the lower frequencies stronger than the higher frequencies. Preferably, it is desirable that all frequencies are at essentially the same level in terms of signal strength, which is known in the communications industry as a flat spectrum. In an optical node application, the signal arrives at the optical receiver basically flat. A linear equalizer is used in the node to provide the desired linearly titled output.
Amplifiers, fiber optic nodes, and head-end equipment are utilized in CATV systems, and most utilize a removable equalizer (cable or linear). As applied to the CATV industry, an equalizer is typically a plug-in device used to adjust the desired slope of a cable signal of an RF broadband signal in amplifiers, fiber optic nodes, and head-end equipment. An equalizer is typically a passive R-L-C (resistor-inductor-capacitor) circuit designed with a response signature that negates, or flattens, a corresponding amount of coaxial cable loss or a linear tilt for various operating frequencies. Referring back to the spectral analogy above, the signal spectrum exiting a length of coaxial cable generally has a stronger signal level at lower frequencies than higher frequencies. The signals are routed through an equalizer of appropriate value for the given cable length. Typically, the equalizer does not adjust the “lower” power or signal levels at the higher frequencies but rather attenuates the “higher” signals found at the lower frequencies to create a flat frequency spectrum. This means that all frequencies are about the same amplitude as a result of being routed through the equalizer.
Previous equalizers, such as the equalizer shown in FIGS. 1A and 1B, generally include a printed circuit board with components (such as resistors, inductors, and capacitors) mounted on the board and contact pins exiting the circuit board. The equalizer plugs in to its host unit, for example, a CATV amplifier, by insertion of the pins into corresponding sockets on the host unit. The friction between the pins and sockets holds the equalizer in place, but the delicate electronic components are left exposed. This is a problem because each equalizer has different operating characteristics and movement of the circuit elements could cause those characteristics to change.
In the electronics industry, it has long been known that one can encapsulate certain active circuit components in plastic or similar materials to add robustness and protection to the circuit. For instance, integrated circuit makers like VLSI and LSI have obtained U.S. Pat. Nos. 5,448,825 and 5,570,272, respectively, for methods and apparatuses for encapsulating integrated circuits. Indeed, the art of encapsulating integrated circuits is quite advanced, with patents being awarded on the particular materials for encapsulating the integrated circuits, such as U.S. Pat. No. 6,030,684, or on materials with particular thermal characteristics, such as the material described in U.S. Pat. No. 5,909,915.
While active, integrated circuits have long been encapsulated, passive circuits like equalizers have not been. The encapsulating material itself would modify the operating characteristics of these circuits by effectively adding capacitance to the circuit. Yet applications for these circuits often require very precise operating characteristics. Indeed, to allow service personnel to adjust an equalizer to match the required field conditions, tunable equalizers were developed with removable, snap-on covers, as shown in FIGS. 2A and 2B. FIGS. 3A–3C show a non-tunable, double-sided equalizer with a snap-on enclosure, although open areas remain between the cover and the substrate holding the circuit elements.
The advantage of such tunable equalizers or covered equalizers is that they allowed for re-tuning of the circuit if necessary and/or provided some protection of the circuit to maintain the equalizer's desired operating characteristics. But disadvantages abounded. The circuit elements remained relatively exposed, the products were harder to manufacture given their small sizes and consequent small profiles. The snap-on covers or enclosures are manufactured separately from the circuit, and attached to the completed circuit at some late stage in the manufacturing of the equalizer. The snap-on cover does not physically touch the circuit components because it would impact the electrical circuit, but the cover often touches the edges of the circuit board. Although providing some cover and protection to the equalizer circuit, the snap-on plastic cover is problematic because the cover can come off or shift physically on the circuit and give the equalizer a flimsy feel. Additionally, if the cover contacts the circuit, it may change the performance of the equalizer.
Accordingly, there is a need for an equalizer that is more robust and easily handled such that the electronic components of the circuit are adequately protected and do not require periodic tuning.